of at least 60 Petrie dishes at each isolation experiment in order to obtain sufficient samples.

The fungus which emerged after severe sterilization was considered likely to be the cause of mycorrhizas since the innermost hyphae of the Hartig net would be less exposed to the sterilizing fluid than would be non-mycorrhizal fungi located on the surface of the mycorrhizas.

The mycorrhizal suspect grew fairly rapidly on agar (Table 1). When isolated under mild sterilization, it competed successfully at times with Penicillium and Verticillium, but occasionally was overgrown by Penicillium. Bacteria frequently emerged with the suspected mycotroph after light sterilization, but in the majority of cases the fungus was dominant.

Young colonies are near-white at first. “Leading” hyphae at the colony margin, growing on the agar surface are widely separate (Fig. 1), but hyphae penetrating the medium grow close together, making slower progress than the surface hyphae. Aerial hyphae do not generally rise more than 2 mm above the substratum. On Malt and Peptone agar aerial mycelium is relatively abundant, rapidly turning grey on the latter medium.

Apical segments of hyphae on agar are broad (about 14μ), smooth, and rounded at the tips. Under high magnification, reticulate thickenings are apparent within the outwardly smooth walls. The protoplast absorbs many stains deeply and evenly. The first septa are usually no closer than 600μ to the tip. Branch hyphae are smaller than the “leaders” (as fine as 2μ) and emerge roughly 450μ behind the tip at right angles. Branch hyphae may enlarge to the size of the “leaders”.

Whereas the tips of surface hyphae on agar are rounded, the extremities of aerial hyphae often taper to a point. The paths of the main surface hyphae on agar are sinuous and over-lapping (Fig. 1).

Most remarkable is the frequent, close twining of fine hyphae around larger hyphae (Fig. 2).

This tendril-like behaviour might be well suited to the ensnaring of roots by a mycorrhizal fungus.

Coils of hyphae aggregated into a mass about 500μ in diameter, are found in older parts of the colony both on aerial and on submerged mycelium.

In colonies which have long overgrown the agar, superficially white circular growths about 2–3 mm in diameter appear on the surface. Their bulk constituent is tightly woven pseudoparanchyma, darkly pigmented in the interior.

Septation of most hyphae increases with age, and dark pigments appear in the walls. Oil globules are abundant.

On some media submerged hyphae become black quite rapidly; but the aerial mycelium remains nearly white for a long period in most cases.

On Prune and plain agar the aging submerged hyphae become brownish pink and eventually black. On Peptone a distinct green colouration of submerged hyphae precedes the eventual blackening.

Old mycelium both aerial and submerged, gives rise to irregular conidia, either by intercalary septation or from hyphal extremities (Fig. 3). Conidia vary from round to rounded oblong.

Anastomosis of hyphae is common (Fig. 4), and is most abundant on hyphae growing over a glass surface.

Apart from the possession of larger, round tipped hyphae, and more rapid growth, the isolate from Nothofagus shows significant resemblance to Mycelium

radicis Fagi (Chan) which has been isolated from mycorrhizas of Fagus sylvatica (Harley, 1939).

Comparisons between the two are listed in Table I. As far as possible the tabulation has been arranged to follow the form used by Harley (1939).

A comparison was made of the growth rates of the Nothofagus isolate with those of Mycelium radicis Fagi (based on radial spread) on different agar media (Tables II (a) and (b)). Each measurement for the respective media was averaged from 10 Petrie dishes, except for KNO₃, in which case 2 dishes were discounted owing to contamination. The original source of test material was from an isolation made on Prune agar. Subsequently the fungus was twice sub-cultured on gelatin-potato-dextrose agar (Kligman, 1943). Inocula of about 8 cu mm were taken at random from the periphery of 3 dishes of similar origin in which hyphae had slightly overgrown the edge of agar.

Growth of the Nothofagus isolate was faster on all media than was that of Mycelium radicis Fagi (Chan) (Harley, 1939). In both fungi, growth was greatest on Peptone. Ammonia produced greater growth than Nitrate with the Nothofagus isolate. The reverse case held for Mycelium radicis Fagi (Chan).